Electrochromic devices include electrochromic materials that are known to change their optical properties, such as coloration, in response to the application of an electrical potential, thereby making the device more or less transparent or more or less reflective. Typical prior art electrochromic devices include a counter electrode layer, an electrochromic material layer which is deposited substantially parallel to the counter electrode layer, and an ionically conductive layer separating the counter electrode layer from the electrochromic layer respectively. In addition, two transparent conductive layers respectively are substantially parallel to and in contact with the counter electrode layer and the electrochromic layer. Materials for making the counter electrode layer, the electrochromic material layer, the ionically conductive layer and the conductive layers are known and described, for example, in U.S. Patent Application No. 2008/0169185, incorporated by reference herein, and desirably are substantially transparent oxides or nitrides. When an electric potential is applied across the layered structure of the electrochromic device, such as by connecting the respective conductive layers to a low voltage electrical source, ions, such as Li+ ions stored in the counter electrode layer, flow from the counter electrode layer, through the ion conductor layer and to the electrochromic layer. In addition, electrons flow from the counter electrode layer, around an external circuit including a low voltage electrical source, to the electrochromic layer so as to maintain charge neutrality in the counter electrode layer and the electrochromic layer. The transfer of ions and electrons to the electrochromic layer causes the optical characteristics of the electrochromic layer, and optionally the counter electrode layer in a complementary EC device, to change, thereby changing the coloration and, thus, the transparency of the electrochromic device.
FIGS. 1A and 1B illustrate plan and cross-sectional views, respectively, of a typical prior art electrochromic device 20. The device 20 includes isolated transparent conductive layer regions 26A and 26B that have been formed on a substrate 34, such as glass. In addition, the device 20 includes a counter electrode layer 28, an ion conductive layer 32, an electrochromic layer 30 and a transparent conductive layer 24, which have been deposited in sequence over the conductive layer regions 26. It is to be understood that the relative positions of the electrochromic and counter electrode layers of the device 20 may be interchanged. Further, the device 20 includes a bus bar 40 which is in contact only with the conductive layer region 26A, and a bus bar 42 which may be formed on the conductive layer region 26B and is in contact with the conductive layer 24. The conductive layer region 26A is physically isolated from the conductive layer region 26B and the bus bar 42, and the conductive layer 24 is physically isolated from the bus bar 40. Although an electrochromic device may have a variety of shapes, such as including curved sides, the illustrative, exemplary device 20 is a rectangular device with the bus bars 40 and 42 extending parallel to each other, adjacent to respective opposing sides 25, 27 of the device 20, and separated from each other by a distance W. Further, the bus bars 40 and 42 are connected by wires to positive and negative terminals, respectively, of a low voltage electrical source 22 (the wires and the source 22 together constituting an “external circuit”).
Referring to FIGS. 1A and 1B, when the source 22 is operated to apply an electrical potential across the bus bars 40, 42, electrons, and thus a current, flows from the bus bar 42, across the transparent conductive layer 24 and into the electrochromic layer 30. In addition, if the ion conductive layer 32 is an imperfect electronic insulator as is the case in many thin film EC devices, a small current, commonly referred to as a leakage current, flows from the bus bar 42, through the conductive layer 24 and the electrochromic layer 30, and into the ion conductive layer 32. Further, ions flow from the counter electrode layer 28, through the ion conductive layer 32, and to the electrochromic layer 30, and a charge balance is maintained by electrons being extracted from the counter electrode layer 28, and then being inserted into the electrochromic layer 30 via the external circuit. As the current flows away from the bus bar 42 across the conductive layer 24 and towards the bus bar 40, voltage is dropped by virtue of the finite sheet resistance of the conductive layer 24, which is typically about 10-20 Ohms/square. In addition, current flowing across the conductive layer 24 is incrementally reduced, as current is drawn through the combination of the layers 30, 32 and 28 (“stack”) to produce the electrochromic coloration in the device 20. Consequently, if the device 20 is considered to be formed from successive adjacent segments arranged between the bus bars 40, 42 and extending between the transparent conductor layer 24 and the conductive layer region 26B, the amount of current flowing through the stack at the segment of the conductive layer 24 closest to the bus bar 40 will be close to zero, as the majority of the current will have passed down through the stack. Assuming that the sheet resistance of the transparent conductive layer 24 is substantially uniform between the bus bars 40 and 42, the voltage drop across the transparent conductive layer 24 extending between the bus bars 40, 42, will be proportional to the current flowing through each successive segment of the device 20. Thus, the rate of voltage drop in the transparent conductive layer with respect to distance away from the bus bar 42 will be at a maximum closest to the bus bar 42 and practically zero close to the bus bar 40. A substantially mirrored image of the current flow occurs with respect to the flow of current from the bus bar 40 across the conductive layer region 26A and toward the bus bar 42, in that the current flow across the device 20 in the conductive layer region 26A increases from the bus bar 40 to the bus bar 42 as a result of contributions from successive segments of the device 20. The difference between the voltage profiles for the conductive layer 24 and the conductive layer region 26A, across the width of the device between the bus bars 40, 42, is the potential difference between the conductive layer 24 and the conductive layer region 26A across the width of the electrochromic device extending between the bus bars 40, 42. The potential difference determines the maximum rate of current flow through each segment from the counter electrode layer 28 to the electrochromic layer 30 causing the device 20 to transform to a colored state and, thus, causing coloring of the device 20. Current will flow at a rate proportional to the potential difference across the segments of the device, provided there is a ready supply of charge, in the form of lithium ions and electrons, to satisfy the requirements. The net result is that a non-uniform coloration is initially produced, with the regions closest to the bus-bars, where the potential difference between the transparent conductors is largest, coloring faster than the region in the middle of the device. In an ideal device, which would not have any leakage current, this non-uniformity will even out as the supply of available charge in the counter electrode layer is exhausted, first closest to the bus-bars, and then in the center of the device, as the electrochromic device attains a fully colored state, thereby yielding uniform coloration across the entire area of the device.
After a voltage is initially applied across the bus bars 40, 42 of the electrochromic device 20, the current flowing through the device 20 will drop towards zero, and thus the voltage drops across each of the transparent conductive layers will also approach zero. Whether the voltage between the conductive layer 24 and the conductive layer region 26A, across the width of the electrochromic device 20 extending between the bus bars 40, 42, will become equal or substantially equal to a constant, such as about the applied voltage, in the fully colored state, thereby ultimately yielding a relatively uniform coloration in the electrochromic device 20, however, depends in part upon the width of the conductive layer 24 and the conductive layer region 26A of the electrochromic device 20 extending between the bus bars 40, 42 across which the current flows and the magnitude of the leakage current through the device.
In large sized electrochromic devices having a construction similar to that of the device 20, where the current flows a relatively large distance, such as in excess of about 40 inches, across the conductive layers of the electrochromic device between the opposing bus bars, non-uniform coloration of the device may persist even at full coloration, because a large and non-uniform voltage drop occurs through the stack across the width of the conductive layers extending from the opposing bus bars. This non-uniform voltage drop is caused by the effects of leakage current through the device, which is typically present in electrochromic devices because of the thin-film construction of the layers of the stack. Leakage current flows through the stack, such that a potential difference variation is created across the width of the electrochromic device extending between the bus bars. If the leakage current is significantly large, the potential difference variation becomes sufficiently large to cause a non-uniform coloration in the electrochromic device that may be visible to the naked eye. The non-uniform coloration in the electrochromic device typically results in a lighter area near a region midway between the opposing bus bars (“middle region”), than at regions of the electrochromic device near the bus bars. In other words, the middle region of the electrochromic device does not experience the same color change, or the same amount of darkening or consistency of darkening, as those regions closer to the bus bars at the sides of the electrochromic device. It is has been observed that when electrochromic devices constructed similar to the device 20 are operated at normal operating voltages, such as between around 2.5V and 4.0V, the leakage current is on the order of 50-500 mA/m2, such that non-uniform coloration across the electrochromic device may become visible to a naked eye when the distance between the opposing bus bars is at least about 30 inches. For typical leakage current levels, color non-uniformity is not readily apparent to the naked eye when the electrochromic device is in the fully colored state and has bus-bar separations less than about 30 inches.
Referring to FIG. 1A, it is highly desirable to position the bus bars 40, 42 very close to the sides 25, 27 of the device 20 to maximize the region of the device 20, which is between the bus bars 40, 42 and, thus, in which coloration can be controlled. Also, by positioning the bus bars near the sides of the device 20, the bus bars, which typically have a thickness of not more than about 0.25 inches, are not visible or are minimally visible, such that the device is aesthetically pleasing when installed in a typical window frame. Large sized electrochromic devices, in which the distance between the bus bars, which typically are at opposing sides of the device, is in excess of about 40 inches, are desirable for many applications, such as a window of an office building or a glass windshield of a car. Thus, in the operation of such large sized electrochromic devices, non-uniform coloration may occur due to the effects of leakage currents, as discussed above, which is not desirable.
One prior art approach for minimizing non-uniformity of coloration in a large sized electrochromic device is to include a bus bar at the central region of the device, in addition to the bus bars disposed at the opposing sides of the device, to form a so-called triple bus bar device. For example, referring to FIG. 2, an exemplary prior art device 200 may include a central bus bar 242 and bus bars 240A and 240B at the opposing sides (“outside bus bars”). The device 200 has a construction of two electrochromic devices 200A, 200B, each being of the type of device 20 shown in FIGS. 1A and 1B, which are connected in parallel, and where the central bus bar 242 is common to both of the electrochromic devices 200A, 200B. Referring to FIG. 2, a first device 20 is disposed adjacent to and in mirror image to a second device 20, such that the bus bars 42 of the respective first and second devices 20 contact each other. The adjacent bus bars 42 are formed into a single, central bus bar 242 of the device 200. The bus bar 242 is connected to the negative terminal of the source 22 for coloring, or alternatively the positive terminal of the source 22 for bleaching, and the bus bars 240A and 240B at the respective opposing sides of the device 200 are connected to the positive, or alternatively the negative, terminal of the source 22. The electrochromic device 200, thus, includes electrochromic devices 200A and 200B that operate in parallel.
If it is assumed that, in the device 200, the bus bars 240A and 240B are separated by the same distance W as the bus bars 40 and 42 of the device 20, each of the parallel devices 200A and 200B individually behaves as if it had a bus bar separation of W/2, leading to a relatively undetectable non-uniformity in the fully colored state. Therefore, when the same voltage is applied to the devices 20 and 200, the voltage difference between the conductive layers at the middle region of each of the devices 200A and 200B is increased in relation to that of the device 20, such that uniform or more uniform coloration may be achieved for the electrochromic device 200 as compared to the device 20 at the same applied voltage.
Although including a central bus bar in an electrochromic device, such as illustrated in the device 200, may result in more uniform coloration for a large sized electrochromic device, the construction of the device 200 with such central bus bar is not desirable. The central bus bar typically is relatively thick, such as about 0.25 inches, and extends across the middle region of the device, which may be a window of a building, thereby being visible to a naked eye as a dark line, which is not aesthetically pleasing. Such thickness of the central bus bar is typical because the central bus bar has to carry all of the current for both halves of the composite device. It is generally desirable, however, that the bus bars of the electrochromic device are deposited to have as narrow a width as possible, so as to allow them to be hidden, such as in the window frames when installed, as much as possible. Consequently, the bus bars themselves have a finite resistance, which may lead to voltage drops along their length during operation of the electrochromic device, which in turn may lead to an end-to-end non-uniformity if significant voltage is dropped in the bus bars themselves. The typical resistance of suitable bus-bar material may be as much as 0.1 Ohm/linear foot, which may lead to significant resistance, and hence voltage drop, when the device is operated to change its optical properties (“switching”) and current is flowing along the length of the bus-bars. In the case of the triple bus bar device, the center bus bar has to carry current for both halves of the device, and therefore will yield twice the voltage drop if it is the same width as the outside bus-bars. In order to minimize the voltage drop, and hence the end-to-end non-uniformity between outside bus bars, it is desirable to make the center bus bar wider than the outside bus bars. Widening the central bus bar, however, will result in an even more undesirable intrusion in the visible area of the electrochromic device.
Also, it has been observed that, in large sized electrochromic devices similar to the device 20, the regions of the device adjacent to the opposing bus bars change color or darken more quickly than at a middle region between the bus bars. Further, it has been observed that these same large sized electrochromic devices may change transmission state (or color) more slowly than electrochromic devices having smaller distances between opposing bus bars. This phenomenon is largely due to the current draw in the larger device being larger, and therefore leading to a larger voltage drop in the transparent conductor layers, thereby reducing the net potential applied to the stack relative to an electrochromic device having a smaller width between opposing bus bars. Also, the slower change in coloration is based, in part, on the application of a voltage to the electrochromic device which is below a maximum level, such as 3V, to avoid overdriving of the electrochromic device at the portions near the bus bars, which may cause damage to the layers of the stack. For example, for a prior art electrochromic device similar to the device 20 having opposing bus bars separated by about six inches, the typical time for the device to change from a full transmission state (fully clear) to a colored state where only five percent of light is transmitted through the device is about 100 seconds, whereas for an electrochromic device similar to the device 20 having bus bars separated by about thirty inches the typical time for obtaining the same coloration change may be about as much as 400 seconds.
Smaller voltage drops across the transparent conductive layers of an electrochromic device also may lead to more uniform coloration during coloration, as well as at full coloration. Therefore, the apparent non-uniformity seen during coloration will be less for an electrochromic device having a smaller width between opposing bus bars than that of a larger electrochromic device.
Therefore, there exists a need for an electrochromic device that is aesthetically pleasing, both in the fully colored state as well as during transition between the colored and clear states, may provide for uniform coloration where current flows over a relatively large distance through the conductive layers of the device, and may provide for a decrease in the time necessary to obtain a desired change in coloration.